A trio of fossilized fish has finally settled an evolutionary conundrum that once puzzled Charles Darwin.

The flatfish has always been regarded as an oddity. Although the immature fish has eyes on opposite sides of its head, one of the eyes migrates around its skull before it reaches maturity. Yet there was no evidence for this development process in the fossil record.

Some evolutionary biologists, including Darwin, have argued that the trait evolved gradually over many generations of flatfish. If true, intermediate flatfish with partially offset eyes would once have lived — but no such fossils have ever been identified, giving succour to both creationists and those arguing for sudden jumps in evolution.

But Matt Friedman, a PhD student studying evolutionary biology at the University of Chicago in Illinois, has now found three examples of these transitional forms. In the process, he unearthed an entirely new species of ancient flatfish in Vienna and re-interpreted already known fossil fish in London. His work is reported in this week's Nature